1. Field of the Invention
The present invention relates to a servo motor control unit for a press machine or bending machine or a other press-forming machine driven by a servo motor.
2. Description of the Related Art
In a press machine or in a bending machine for bending a metal material, etc., normally oil pressure is used as the drive source for opening and closing the die, but there is also known an electric servo press which uses a servo motor to drive a die fastening member.
In this electric servo press, use is made of a method of controlling a position of a movable die while machining a workpiece and a method of controlling a position of a movable die and, when a workpiece and a movable die abut each other and start a press operation, restricting an output of a servo motor for driving the movable die to maintain a constant pressure in control while machining the workpiece.
However, we searched for patent documents relating to the control of pressure in a press machine, but could not find any press machine technology relevant to the present invention.
In an electric servo press, if simply performing position control to machine a workpiece, since the press pressure is not controlled, good precision machining is not possible. Further, even if setting a torque limit to restrict an upper limit of the press pressure and performing the press machining or bending by open control of the pressure, since the pressure actually applied to the workpiece is unknown, high precision machining is not possible. If providing an electric servo press with a pressure sensor to detect the pressure applied to the workpiece and performing pressure feedback control, the problems arise that the switching from position control to pressure control is difficult and the transition from position control to pressure control cannot be achieved smoothly.